1. Field of the Invention
The present invention relates to a burst mode optical transmitter circuit for converting burst data into an optical signal to be transmitted.
With recent developments in multimedia communication techniques, optical communication of high speed and large capacity has been realized. In such an optical communication system, there are continuous data communication and burst data communication in a fiber-optic subscriber system. The present invention relates to a burst mode optical transmitter circuit for converting burst data into an optical signal to stably transmit the data.
2. Description of the Related Art
FIG. 15 is a diagram, for explaining a prior art example, in which 101 represents a semiconductor laser, 102 a monitoring photodiode, 103 a current-to-voltage converting circuit (I/V), 104 an automatic power control (APC) amplifier, 105 a driving circuit, 107 a holding circuit, D a diode, and C a capacitor.
The driving circuit 105 supplies a driving current to the semiconductor laser 101 according to input data DATA in the burst mode. The optical output of the semiconductor laser 101 is transmitted through a not-shown optical fiber to a receiving side. A part of the optical output is detected by the monitoring photodiode 102. The detected output is converted by the current-to-voltage converting circuit 103 into a voltage and is input into the APC amplifier 104. The APC amplifier 104 compares the signal output from the current-to-voltage converter 103 with a reference value and applies a signal corresponding to the difference obtained by the comparison to the holding circuit 107.
The holding circuit 107 includes a diode D and a capacitor C to constitute a peak hold circuit. The value held by the capacitor C is applied as a current control signal to the driving circuit 105 whereby, when the optical output from the semiconductor laser 101 becomes larger than a predetermined value, the driving current supplied from the driving circuit 105 to the semiconductor laser 101 is decreased and, in contrast, when the optical output from the semiconductor laser 101 becomes smaller than a predetermined value, the driving current from the driving circuit 105 to the semiconductor laser 101 is increased. Thus the optical output is stabilized.
During an interrupt period of the data DATA input in the burst mode, the current control signal is held in the capacitor C in the holding circuit 107. Accordingly, even when the data DATA is input, after the interrupt period, the output of the semiconductor laser 101 is immediately controlled to have a predetermined level so that an optical signal can be transmitted. That is, the holding circuit 107 holds the current control signal to be input into the driving circuit 105 during the interrupt period of the burst data.
The semiconductor laser 101 has a light output characteristic with respect to the driving current as shown in FIG. 16. Because of this fact, the optical output can be stabilized as mentioned before. However, the light output characteristic is such that when the ambient temperature T or the temperature of the semiconductor laser 101 itself is lowered to be T1 which is lower than T2 as illustrated, the optical output is greatly increased even when the driving current is the same. To cope with this, a temperature compensating circuit to compensate for the temperature characteristic has been proposed to be implemented into the circuit.
When burst data is converted into an optical signal and is transmitted, cases where the interrupt period of the burst data is relatively long may often occur. For example, when data DATA is input according to the burst mode as shown in FIG. 17, the LD current, i.e., the current to drive the semiconductor laser, is controlled by the above-mentioned control so that the optical output is made constant.
Then, when data is input again after an interrupt period of the data DATA, the ambient temperature may be changed, for instance, from day temperature to night temperature and so forth. In particular, when the temperature is lowered, and when the driving current is supplied from the driving circuit 105 to the semiconductor laser 101 based on the current control signal held in the holding circuit 107, there is a problem in that the optical output from the semiconductor laser 101 is greatly increased so that the semiconductor laser 101 is destroyed.
In another prior art, an APC circuit is provided in which an optical output from a semiconductor laser is monitored to be sampled and held, the held value is compared with a reference value, and the result of the comparison is input into a control voltage supplying unit. In such an APC circuit, it has been proposed to provide a temperature compensating current generating unit and a temperature compensating voltage generating unit as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-260720. In this case, the current corresponding to the ambient temperature is generated from the temperature compensating current generating unit and is input into the temperature compensating voltage generating unit by which the current is converted into a voltage which is then input to the control voltage supplying unit. This APC circuit, however, has a problem of complex construction and therefore of being expensive.
In still another prior art, there has been provided an APC circuit in which an optical output from a semiconductor laser is monitored to be sampled and held, the held value is compared with a reference value, and the driving current of the semiconductor laser is controlled by the compared result. In this APC circuit, the sample and hold circuit is reset at the time of turning on or off of the power supply so as to prevent an erroneous operation of the APC circuit, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-131675. This prior art, however, does not relate to the transmission of burst data, and the above-mentioned problem which is caused when the temperature is changed in an interrupt period of the burst data, and the means to solve the problem are not disclosed in this prior art.